Certain plants, of which alfalfa, soybeans, and common beans are examples, suffer from disease conditions called "damping off" and "root rot." The symptoms of damping off include the desiccation and subsequent death of seedlings soon after germination. Root rot symptoms include chlorosis and wilt of leaves and yellow to brown lesions with diffuse margins on roots and stems. The lesions can eventually lead to girdling and subsequent root decay resulting in decreased robustness in the plant or even in death. Often plants suffering from root rot begin by showing such symptoms, which may be mistaken as symptoms of drought and starvation. Such plants may be more vulnerable than healthy plants to attack by other pathogens, which are then mistaken as the cause of the death of the plants.
Damping off and root rot are merely two different sets of symptoms caused by infection of the plant by the same fungi and, in particular, by members of the Phytophthora, Pythium, Aphanomvces, Rhizoctonia, and Fusarium genera. Thus, Phytophthora megasperma f. sp. medicaginis (now formally known as Phytophthora medicaginis, and referred to hereinafter as "Pmm") causes both damping off and root rot in alfalfa when soils are wet in most parts of the world where alfalfa is grown, and Phytophthora megasperma f.-sp. glycinea has been shown to cause root rot in soybeans under wet growing conditions. However, fungi from among the other genera listed also are believed to attack alfalfa and soybeans. Root rot in common beans is believed caused by a complex of fungi including members of more than one of the genera referred to.
In general, control of damping off and root rot has been attempted by breeding for resistant plants. However, completely resistant cultivars have not been developed such that damping off and root rot remain major causes of crop loss. This is especially true under chronically wet growing conditions or when the same crop is planted repeatedly in the same fields. Certain fungicides such as metalaxyl partially control root rot. However, such fungicides are fairly expensive. For some crops, such as alfalfa, their use is not economically feasible. Also, resistance of the fungi to the fungicides can develop rapidly.
"Biological control" is defined as pathogen control by the use of a second organism. Mechanisms of biological control are diverse. For example, certain enteric bacteria have been examined for their usefulness in biological control of root rot in alfalfa. It is believed that control is obtained by competition between the enteric bacteria and the fungi for space on the surface of the alfalfa roots. In contrast, a toxin produced by one species of bacteria may be used to control another species of bacteria that appears as a pathogen. Bacterially produced antibiotics are an example of such toxins. The toxin can be isolated from the species producing it and administered directly, as is the common procedure with penicillin, or the species itself m ay be administered under appropriate circumstances to produce the toxin in situ. Once identified, such toxins produced by soil-dwelling bacteria may have utility in diverse other areas as antifungal or antibiotic agents.